1. Field of the Invention
The invention relates generally to systems and methods for monitoring conditions within a plugged subterranean wellbore.
2. Description of the Related Art
At the end of the production lifetime of a well, the wellbore is permanently plugged and abandoned. The typical fashion of doing this involves milling away and removing an upper portion of the surface casing (usually about 500 feet worth of casing) and then setting one or more completion plugs in casing below the milled away area. Thereafter, the uncased portion of the wellbore above the completion plugs is filled with cement (a “surface cement plug”).
This procedure is usually intended to be a permanent closing off of the wellbore. However, it is often important to obtain information from below the surface cement plug, often on a continuous basis. This information might include pressure, temperature (above or below the surface cement plug) or information about oil and gas migration though the cement. Continuous monitoring of such parameters can help identify potential breach of the plugged and abandoned state. The frequency of such communication may be low. In some cases, communication might only be required if measured parameters exceed a predetermined threshold value.
Some methods for transmitting information from a downhole location have involved the use of conductive casing or tubing for transmission. Those methods are unsuitable for use in most instances where a well is plugged off since there is no casing or tubing which traverses the surface cement plug.